This invention relates generally to apparatus for closing and sealing a doorway in a wall. More specifically, the invention relates to apparatus for closing and sealing a doorway which leads to the chamber of a furnace such as a vacuum furnace for brazing aluminum parts. A furnace of this general type is disclosed in Bielefeldt U.S. Pat. No. 3,609,295. During a typical process for brazing aluminum parts in a vacuum furnace, magnesium is vaporized in the furnace chamber in order to promote wetting of the aluminum surfaces and avoid the need for using flux. Moreau U.S. Pat. No. 3,673,678 discloses a fluxless brazing process in which magnesium is vaporized in the furnace chamber.
The apparatus for closing and sealing the doorway of the furnace chamber usually comprises a door which is mounted on the inner side of a door carrier to move with the carrier and also to move relative to the carrier. The carrier and the door are adapted to be shifted back and forth in unison along a path substantially paralleling the plane of the doorway to move the door between open and closed positions relative to the doorway. After the door has been moved to its closed position, it is shifted inwardly relative to the carrier and into tight engagement with the wall of the furnace chamber so as to seal the doorway. The door is shifted outwardly from sealing engagement with the wall prior to being moved to its open position.
Bearing means engage the outer side of the carrier to support the carrier with rolling friction as the carrier moves the door between its open and closed positions. The bearing means also sustain the outward reaction force which is imposed on the carrier when the door is shifted inwardly into sealing engagement with the furnace wall.
In prior door apparatus of the foregoing type, the bearing means have consisted of a ball bearing roller mounted to rotate about a fixed shaft. Rollers of this type experience a very short service life. The bearings become crushed by the outward force exerted by the carrier and, in addition, magnesium which accumulates in the vicinity of the furnace causes the bearings to gall and jam.